Re: [RFC PATCH] vfs: add MNT_NOFOLLOW flag to umount(2)

[Bodo Eggert]

On Fri, 5 Feb 2010, Miklos Szeredi wrote:

> On Thu, 04 Feb 2010, Bodo Eggert wrote:
> > Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> >
> > > Additionally, return -EINVAL if an unknown flag is encountered.  This
> > > makes it possible for the caller to determine if a flag is supported
> > > or not (at least on kernels with this patch).
> >
> > There should be a guaranteed-to-be-invalid flag or flag-combination in
> > order to safely detect this feature.
>
> It's difficult though, because the app would have to make sure the
> detection itself would reliably fail, and with a different error.
>
> Simply checking the kernel version might be easier.

Yes, that's exactly what I intend to make possible:

| ret = umount("/tmp/mkstmp", UMOUNT_IMPOSSIBLE_FLAG);
| if (ret != -1 || errno != -EINVAL)
|     goto extended_mount_flags_are_not_supported;

Checking the kernel version is a bad idea, because the application might 
be portable, or features might be backported. ¢¢